This invention relates to an improved fiber optic cable. More particularly, it relates to the use of pre-stretched strength members parallel to optical fibers for providing a force equal to or greater than the predetermined break strength of an optical fiber.
With the realization of the limited number of communication channels available in metal conductor electrical communication cable, the use of optical fibers with modulated light sources for communication is fast becoming a reality. An optical fiber provides at least one order of magnitude more channels per tranmission path than a metal conductor.
One of the major problems in the use of optical fibers is that they are normally made of glass and thus are easily broken. One type of breakage occurs when the cable, and thus optical fiber, is placed under a longitudinal load, causing longitudinal stress and strain on the fiber. Several means have been devised to overcome this problem of longitudinal stress. U.S. Pat. No. 3,855,897 shows a plurality of optical fibers arranged in a helical fashion about the core of the cable. During longitudinal stress, the pitch in each helical turn will lose amplitude to absorb the stress on the fibers rather than the fibers themselves.
U.S. Pat. No. 3,883,218 also discloses the use of a helical wound fiber bundle which also takes up stress during longitudinal load on the cable. Furthermore, this patent discloses a reinforcing member inserted in the center of the fiber bundle for providing strength to the cable as well as overcoming certain stress problems.
The use of helical fibers pose severe manufacturing problems as well as impaired light transmission.
U.S. Pat. No. 4,009,932 discloses the use of strength members parallel to the optical fiber in a optical cable core. The strength members are made of metal wires which have a much higher break strength than the fiber. The use of these metal strength members increases the cost of the resulting cable. Furthermore, the metal wire acts as an antenna which could cause electrical interference with the electronics at both ends of the cable.
While all of these above patents disclose means of protecting optical fibers from longitudinal stress, there is a need to provide a simple and relatively inexpensive means of protecting optical fibers.